There are many applications of illuminators and displays where the angle of light output or the display viewing angle is required to be controlled or limited. For example, in automotive applications of displays, it is desirable to restrict the vertical output angle of light from the display so as to prevent undesirable reflections from a vehicle windscreen in order to reduce or avoid driver distraction. Also, light propagating generally downwardly, for example towards the floor, is wasted and represents a reduction in efficiency of light utilisation.
US Re 27617 discloses a known type of louvred film for controlling the output angle of light from a display. The film comprises alternating transparent and opaque layers in an arrangement which is similar to a Venetian blind. Such an arrangement passes light which is propagating in a direction parallel and nearly parallel to the planes of the layers but absorbs light propagating at larger angles to these planes. Thus, light propagating in undesirable directions is absorbed and this reduces the efficiency of light utilisation.
JP 2004/245918 discloses a similar type of louvered film but modified to comprise transparent, reflecting and opaque layers. Again, light propagating parallel and nearly parallel to the louvre planes is transmitted. Light propagating at higher angles to the planes and generally upwardly is reflected downwardly into a non-viewing region whereas light propagating at higher angles generally downwardly is absorbed. Again, the efficiency of light utilisation is relatively low.
U.S. Pat. No. 6,091,547 discloses the use of a prismatic film, which collects light from “off-axis” directions and redirects it into “on-axis” directions. This improves the efficiency of light utilisation by redirecting light, which would otherwise be wasted, into a display viewing region. However, substantial amounts of light are still transmitted outside the viewing region.
WO 05031412 discloses a planar waveguide forming part of a backlight of a three dimensional display. The waveguide receives light from one of its edges and the light propagates within the waveguide by total internal reflection. A series of grooves is cut into an output surface of the waveguide to permit light to exit the waveguide. The grooves act as parallel spaced line sources of light.
U.S. Pat. No. 4,726,662 discloses a display which selectively scatters or transmits light in response to an appropriate input. A light source is disposed behind the display device and includes a prismatic lens for focusing ambient transmitted light onto a light-absorbing surface so as to enhance display contrast.
US 2003/0035231 discloses an optical film for a reflective display. The film is disposed behind a light-modulating layer of the display and comprises a prismatic reflective structure.